The focus of this project is to determine the location and mechanism of an inherited retinal degeneration using as a model system rod-cone dysplasia in the Irish setter. This is one of a group of retinal degenerations collectively termed "progressive retinal atrophy". It was known from early experiments that cGMP levels were extremely high in affected whole retinas and since it is the outer segment (OS) layer that normally contains most of the cGMP, it was assumed that this was the location of the cGMP in this disease state also. We have now shown that it is the opposite end of the photoreceptor cell, the outer plexiform layer, that contains the bulk of the retinal cGMP in diseased retinas. At the time of the peak levels (28 days), cGMP is 16-fold higher in diseased vs normal OPL. Levels remain high for at least 7 weeks before dropping, presumably as a result of photoreceptor cell degeneration. Neither masked light or EM microscopy can detect changes between normal and dystrophic retinas before 13 days, yet we can see a clear difference in cGMP profiles across the retinal layers as early as 11 to 12 days when there is a 6-fold difference in cGMP in the OPL. However, a much greater, 23-fold increase in cGMP levels in the OPL occurs in dystrophic retinas between 13 and 20 days, the time at which diseased retinas appear to be blocked in their normal differentiation process. We are now focusing on measuring guanylate cyclase activity in retinal layers, particularly the OPL, to see if it is the increased activity of that enzyme is responsible for the large increases in cGMP that we see.